1. Field of the Invention
The present invention relates to an organic photoreceptor and an electrophotographic image forming apparatus including the photoreceptor. More particularly, the invention relates to an organic photoreceptor having the same advantages as a conventional laminated photoreceptor but improved electric properties such as higher photosensitivity and lower exposure potential. The invention is also directed to an electrophotographic image forming apparatus including the photoreceptor.
2. Description of the Related Art
A two-layered organic photoreceptor used in an electrophotographic process includes a charge transporting layer (CTL) and a charge generating layer (CGL) placed on an electrically conductive substrate. Two-layered organic photoreceptors, are laminated organic photoreceptors and can be classified into positively charged organic photoreceptors, in which a CTL is formed on an electrically conductive substrate and a CGL is formed on the CTL to positively charge the surface of the photoreceptor, and negatively charged organic photoreceptors, in which a CGL is formed on an electrically conductive substrate and a CTL is formed on the CGL to negatively charge the surface of the photoreceptor.
The positively charged organic photoreceptor having the above described structure forms an electrophotographic image in the following manner.
First, when a surface of the organic photoreceptor is charged positively and a laser beam is radiated thereto, positive and negative charges are generated in the CGL. The positive charges are injected into the CTL by an applied electric field and move to the electrically conductive substrate, while negative charges (electrons) move to a surface of the CGL (that is, to a surface of an overcoating layer if an overcoating layer is coated on the surface of the CGL) and neutralize the surface charges. Consequently, the surface potential of the exposed portion is changed, and a latent image according to the changed surface potential is formed. When toner is developed on the latent image, a toner image is formed on the surface of the organic photoreceptor. The toner image is transferred to a surface of a receptor such as paper or other transferring medium.
In the case of the negatively charged organic photoreceptor, when a surface of the organic photoreceptor is charged with negative charges and a laser beam is radiated thereto, positive and negative charges are generated in the CGL. Then, the negative charges move to the electrically conductive substrate by an applied electric field, and the positive charges move to the CTL to neutralize the surface charges. As a result, the changed surface potential of the exposed portion is changed, and a latent image according to the changed surface potential is formed. When toner is developed on the latent image, a toner image is formed on the surface of the organic photoreceptor. The toner image is transferred to a surface of a receptor such as paper or other transferring medium.
Since the functions of the CTL and the CGL are different from each other, electric properties such as the charge potential and the exposure potential of the two-layer organic photoreceptor can be achieved more easily compared to a single-layered type organic photoreceptor where a series of electric properties have to be achieved using one single layer. Moreover, an electric field can be stably applied to a laminated organic photoreceptor even when the coating thickness of the CGL and the CTL is smaller. Accordingly, the laminated organic photoreceptor can keep more charges than a single-layered organic photoreceptor even when an electric field of the same intensity is applied, and thus a greater amount of toner can be developed on the surface of the organic photoreceptor. Accordingly, not only dry toner but also to wet toner can be used with the positive charge laminated organic photoreceptor.
However, the amount of a binder should be increased when coating a composition for forming a CGL on the electrically conductive substrate or a CTL in the laminated organic photoreceptor in order to increase the stability of the CGL coating solution, the coating quality of the CGL during coating, and the adhesion property between the CGL and the electrically conductive substrate or the CTL. However, when the binder amount in the composition for the CGL is great, the stability of the coating solution for the CGL, the coating quality, and the adhesion property are improved, but electric properties are significantly deteriorated. For example, the photosensitivity is decreased because electrons cannot be easily transported in the charge generating layer and the exposure potential is increased. Particularly, when the thickness of the CGL is increased to obtain high photosensitivity, charges are mainly generated in the upper portion of the CGL and thus electrons generated in the CGL have difficulty to efficiently move to the electrically conductive substrate or the CTL. Thus, the charges are not easily generated, the photosensitivity is decreased, and the exposure potential is increased.